1. Field of the Invention
The present invention relates generally to the art of rigid urethane foams and more particularly to isocyanurate modified polyurethane foams in which at least a portion of the polyol component includes a digestion product of polyalkylene terephthalate residue or scrap dissolved in one or more organic polyols.
2. Description of the Prior Art
Rigid polyurethane foams are well known and are commonly prepared from organic polyisocyanates and organic polyols together with known blowing agents, surfactants and catalysts for the reaction of --OH and --NCO radicals. Such foams are used in construction, refrigeration and insulation because they may be prepared in a wide variety of densities and because they are closed cell foams. One of the major factors contributing to the failure of such foams to reach large scale commercial acceptance is that the basic foam systems have high smoke and flame generation ratings when evaluated by ASTM E84. For fire retardant applications, it has been customary to employ halogenated additives and/or halogenated organic polyols. Several problems result from the required use of substantial amounts of halogenated polyols, not the least of which is the greatly increased cost of these fire retardant foams. Further, the toxicity of halogen containing gases which result from the incomplete combustion of urethane or isocyanurate modified urethane foams containing such halogenated materials is a matter of concern. A need exists for low smoke and flame spread polyurethane foams which do not require substantial amounts of such halogenated materials.
One proposed solution to the problem is described in commonly assigned U.S. Pat. No. 4,223,068, issued Sept. 16, 1980 to Carlstrom, et al. for "Rigid Polyurethane Foam Containing Polyester Residue Digestion Product and Building Panel Made Therefrom." In this patent, the rigid polyurethane foam is produced from polyisocyanates and polyols wherein from 5-30% by weight of the polyol ingredient is the digestion product of polyalkylene terephthalate residues or scrap in organic polyols. Urethane foams prepared using such digestion products exhibit uniform density when compared to similar foams which do not contain such products and also show equivalent or superior physical properties. Moreover, and quite unexpectedly, such foams have lower flame spread and smoke generation ratings than corresponding foams prepared without such digestion products. The properties are unexpected in these particular applications because the polyol digestion products are primarily linear diols and would not be expected to yield the strength properties which they in fact do exhibit. Such digestion products minimize the need for more costly halogenated materials and further are advantageous in that they are generally less expensive than virgin polyols used in polyurethane foam preparation.
The 5-30% limitation expressed in the aforementioned Carlstrom patent represents the approximate range for inclusion of such digestion products in typical urethane foam compositions. Higher amounts were found to lead to deterioration of foam properties.
The digestion products according to the aforementioned Carlstrom patent are prepared from polyalkylene terephthalate (PET) scrap which is readily available from photographic films, synthetic fibers and from PET beverage bottles, among other sources. The starting material is also available from sludges obtained as by-products in polyalkylene terephthalate manufacturing plants. Such scrap usually has a molecular weight in the range of 15,000-100,000. The digesting polyols described as being useful by Carlstrom, et al. are those aromatic or aliphatic polyols having a molecular weight of 500 or less and include ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, polyethylene glycol, dipropylene glycol, polypropylene glycol and other ethylene glycols and glycol ethers, hydroxy-terminated polyesters, bis(2-hydroxyethoxyethyl)glutarate and bis(2-hydroxy-ethyl)terephthalate. The digestion reaction is carried out at elevated temperatures, such as 200.degree.-250.degree. C., for several hours and under a nitrogen atmosphere to prevent oxidation reactions. The digested product is cooled and the product is used directly or stored for subsequent use.
Foams prepared using such digestion polyols may be used, for example, in building panels where insulation and fire retardency are important requisites. Ratings of 25 or less flame spread and 100 or less smoke generation have been obtained for building panels having two incombustible skins using ASTM E84.
Recently, another type of foam has been investigated by various corporate and academic institutions, i.e. the isocyanurate foams. These foams include trimer units formed by the cyclization of the isocyanate (--N.dbd.C.dbd.O) radicals catalyzed by certain types of oxides, alkoxides, amines, carboxylates, hydrides, and hyroxides of quarternary nitrogen, phosphorus, arsenic and antimony. The basic chemistry of monomeric isocyanurates and the preparation of isocyanurate foams is described in an article "Isocyanurate Foams: Chemistry, Properties and Processing" by Reymore, et al. and published in the Journal of Cellular Plastics, Nov/Dec 1975, pp. 328-345. The isocyanurate rings exhibit a high degree of thermal stability, a property which researchers believe can be exploited in the foam field. However, initial research with isocyanurate foams met with failure because the products were extremely friable, probably because of the high cross-link density. Moreover, humid aging properties were quite poor. Various attempts have been made to modify the properties of isocyanurate foams by varying the NCO index in foam preparations as well as by modifying the ring forming or polyol components of the final foam. It is only in recent years that substantial progress has been made in achieving the desired results. Polyol substitution has been attempted, but typically polyol addition results in a decrease in heat stability and an increase in flammability, although the physical properties may be more precisely tailored using polyols.
It has also been recognized as advantageous to use lower cost polyols in the preparation of polyisocyanurate systems. For example, Hughes and Clinton presented a paper entitled "Development of Lower Cost Polyurethane Modified Polyisocyanurate and Polyurethane Foams" at the 25th Annual Urethane Division Technical Conference sponsored by the Society of the Plastics Industry in Scottsdale, Ariz. on Oct. 29, 1979. The work described in this paper included the replacement of conventional polyols with lower cost polyols such as Urol-11 and the Terate polyols. Urol-11 has a hydroxyl number of 400-460, an acidity percent of 0.2, a viscosity (cps at 25.degree. C.) of 5000 and a moisture (wt %) of 0.3 and had previously been employed as a low cost polyol for one-shot polyurethane foam applications. This material has a functionality of about 3 and a molecular weight of about 400.
The Terate polyols are moderate viscosity, aromatic polyester polyols derived from polycarbomethoxy-substituted diphenyls, polyphenyls, and benzyl esters of the toluate family and are manufactured and sold by Hercules Incorporated of Wilmington, Delaware. Hughes and Clinton attempted to improve polyisocyanurates foams with Terates, recognizing that Hercules marketed the Terates as additives useful for enhancing fire retardancy (primarily because of the high aromatic content of the Terates.) It was concluded by these authors that low cost materials such as the Urol-11 and Terate materials could be employed in isocyanurate foam systems without reducing the desired physical properties and at the same time maintaining or enhancing the fire retardancy and smoke generation properties of the resultant foam. The one problem that was encountered in this work was the incompatibility of the Terates with the fluorocarbon blowing agents employed. Balancing with proper surfactants seemed to alleviate this problem and Hercules has now introduced new members of the Terate family for use in polyurethane, quasi-trimer and isocyanurate foams which are claimed to be free of the incompatibility problem. The current literature advocates phosphorus addition (0.75-1.0%) in these foams and an isocyanate index as low as 1.60 when Terates are used at high levels as the polyol foam component.
While the prior art described above demonstrates progress in the development of lower cost, flame retardant polyurethane and isocyanurate modified urethane foams having desirable physical properties, all of the problems of the prior art have not been overcome from the standpoint of system reliability and reproducability. The aforementioned incompatability problems and the sensitive catalyst mechanisms which are involved in the preparation of polyisocyanurates have slowed progress. The development of additional isocyanurate modified urethane foams utilizing low cost polyols and which are able to achieve Class I or II ratings for fire retardancy and which have low smoke generation while maintaining physical properties would be a further significant advance in the art.